2.3.1 Morphological studies
The performance and lifetime of a PLED are critically dependent on the properties of active materials and quality of their interfaces, and therefore the surface morphology of PFONPN01 polymer thin films was explored using atomic force microscopy (AFM) images. Fig. 2.2 shows the AFM images of the PFONPN01 thin films heated at various temperatures for 30 mins, ranging from 60 oC to 100 oC (Fig. 2.2 (a)–(c)) and DBT doped PFONPN01 thin films heated at 80 oC (Fig. 2.2 (d)), deposited using a spin coating method.
The thin films had very smooth and amorphous surfaces, with a root mean square (RMS) roughness of 1.573 nm, 0.550 nm and 0.774 nm for the 60 oC, 80 oC and 100 oC heated films respectively. PFONPN01 was then doped with the DBT molecule and the AFM images of the thin film (heated at 80 oC) show no aggregation, which means that DBT is dispersed homogenously in PFONPN01. The DBT doped PFONPN01 thin film shows a
root mean square (RMS) roughness of 0.447 nm.80 oC annealing was selected for device fabrication because at this annealing temperature, the emissive layer film morphology was found to be the best.
Fig. 2.2 AFM images of the PFONPN01 thin films heated at various temperatures, (a) 60
°C, (b) 80 °C, (b) 100 °C and (d) DBT doped PFONPN01 thin films heated at 80 °C.
2.3.2 Blue PLEDs
The blue emitting host plays a significant role in determining the properties of a host–guest system emitting white light. The efficiency of such a system is highly affected by the electron and hole transporting properties of the polymer host as well as the efficiency of charge injection from the cathode side. Here, single layer PLEDs were fabricated using pure PFO and PFONPN01 as the emissive layer (EML) in the ITO/PEDOT:PSS (40 nm)/
PFONPN01(80 nm)/LiF(1.0 nm)/Al(100 nm) (device B1) configuration. Fig. 2.3 (a) and Fig. 2.3 (b) shows the current density vs. voltage (J–V) and brightness vs. current density (B–J) curves of the single layer devices respectively. From these curves, it was observed that the maximum brightness value of device B1 improves significantly to 5636 cd m-2 as compared to 2328 cd m-2 of PFO (more than 2 times).
Fig. 2.3 Current density vs. voltage (J-V) (a) brightness vs. current density (B-J) and (b) curves of the single layer Blue PLEDs.
Fig. 2.4 Current density vs. voltage (J-V) curves of PFO and PFONPN01 based hole only and electron only devices, film thickness = 80 nm.
To understand this improvement in the device properties, an electron dominated device using structure ITO/Al (40 nm)/polymer (80 nm)/LiF(1.0 nm)/Al(100 nm) and an hole dominated device using structure ITO/PEDOT:PSS(40 nm)/ polymer(80 nm)/MoO3(15 nm)/Al(100 nm) for both PFO and PFONPN01 were fabricated and the curves are shown in Fig. 2.4. The hole current in the case of PFONPN01 is found to be little less as compared to that of PFO, however, there has been more than two-fold improvement in electron current in the case of PFONPN01 as compared to that of PFO.
Therefore, the high device performance of PFONPN01 as compared to that of PFO can be
0 2 4 6 8 10 12 14 16
0 50 100 150 200 250
Current Density (mA/cm2)
Voltage (V)
PFO
PFONPN01 (Device B1)
0 50 100 150 200 250
0.0 2.0k 4.0k 6.0k
Brightness(cd/m2)
Current Density (mA/cm2) PFO
PFONPN01 (Device B1)
(a) (b)
0 2 4 6 8 10 12 14
0 2x102 4x102
6x102 PFONPN01 electron only PFO electron only PFONPN01 hole only PFO hole only
Current Density(mA/cm2 )
Voltage (V)
attributed to the enhanced electron transport properties of the PFONPN01 copolymer due to the high electron affinity of the NPN moiety.
To further improve the device performance, multilayer PLEDs using Alq3 as the electron transporting layer were fabricated. The thickness of the Alq3 layer was varied for optimum device performance (5 nm, 10 nm and 20 nm for devices B2, B3 and B4 respectively). Fig. 2.5 (a) and Fig. 2.5 (b) shows the current density vs. voltage (J–V) and brightness vs. current density (B–J) curves of the multilayer devices respectively. The luminous efficiency vs. current density (LE–J) curves of all the single as well as multilayer devices are shown in Fig. 2.6 and the key device properties are summarized in Table 2.1.
The maximum brightness value and LE of multilayer devices B2, B3 and B4 are found to be 9649 cd m-2 and 4.27 cd A-1, 11479 cd m-2 and 4.63 cd A-1 and 11662 cd m-2 and 4.87 cd A-1 respectively.
Fig. 2.5 Current density vs. voltage (J-V) (a) brightness vs. current density (B-J) and (b) curves of the multilayer Blue PLEDs.
Fig. 2.6 The luminous efficiency vs. current density (LE–J) curves of blue PLEDs.
0 2 4 6 8 10 12 14 16
0 50 100 150 200 250
Current Density (mA/cm2 )
Voltage (V)
Device B2 Device B3 Device B4
0 50 100 150 200 250
0.0 3.0k 6.0k 9.0k 12.0k
Brightness(cd/m2)
Current Density (mA/cm2) Device B2
Device B3 Device B4
(a) (b)
0 50 100 150 200 250
0.0 1.0 2.0 3.0 4.0 5.0
Luminous Efficiency (cd/A)
Current Density (mA/cm2) PFO Device B1 Device B2 Device B3 Device B4
Table 2.1 Key device properties of the fabricated blue PLEDs.
aMaximum value.Values in the bracket are average value of 10 devices and standard deviation.
From these results, it was observed that the maximum brightness value of devices B2, B3 and B4 is almost two times that of device B1. The increase in this device brightness can be attributed to the addition of Alq3 as the ETL as well as the better electron injection from the Alq3/LiF/Al cathode. The difference in hole mobility between PFONPN01 and Alq3
results in hole accumulation at the EML–ETL interface. As a result, shifting of the recombination zone away from the cathode occurs. Also, the thin layer of LiF, deposited between Alq3 and Al, reacts with Al and forming Li atoms and AlF3 compounds. The Li atoms then donate electrons to Alq3 molecules resulting in the formation of Alq3 anions that effectively increase electron injection by forming good contact with Al cathodes,33–35 thereby increasing the device brightness. The ETL thickness is also found to affect the device performance by controlling the relative distribution of electrons and holes in the active layer. The brightness value is found to increase with increasing ETL thickness from 5 to 20 nm. Device B4 with an ETL thickness of 20 nm is found to exhibit better properties among all the devices and hence an ETL thickness of 20 nm is used for making WPLEDs.
2.3.3 White PLEDs
Fig. 2.7 (a) shows the solid-state photoluminescence (PL) spectra of PFONPN01 and the DBT chromophore used for realizing white light. The emission from PFONPN01 covers the range of 400–550 nm whereas the emission from the DBT molecule covers the range of 500–650 nm of the visible region, thus making them a suitable combination for realizing white light. However, in order to confirm energy transfer from PFONPN01 to the
Device Onset Voltage
(V)
Maximum Brightness
B (cd/m2)
Luminous Efficiency LEa (cd/A)
Power Efficiencya PE (lm/W)
CIE coordinates
(x,y)
PFO 6.4
(6.48, 0.13)
2328 (2284, 51.87)
1.13 (1.10, 0.03)
0.25 (0.22, 0.03)
0.15, 0.14
B1 7.2
(7.26, 0.05)
5636 (5599, 29.95)
4.21 (4.16, 0.06)
0.94 (0.86, 0.05)
0.15, 0.22
B2 7.3
(7.42, 0.11)
9649 (9488, 132.8)
4.27 (4.17, 0.09)
0.96 (0.89, 0.04)
0.16, 0.22
B3 6.8
(6.98, 0.18)
11479 (10951, 513.3)
4.63 (4.47, 0.13)
1.04 (1.00, 0.03)
0.15,0.23
B4 7.1
(7.18, 0.08)
11662 (11286, 308.8)
4.87 (4.78, 0.06)
1.10 (1.05, 0.03)
0.16, 0.23
DBT chromophore, the solid-state UV-visible absorption spectrum of the DBT molecule was also recorded and is shown in Fig. 2.7 (b) along with the PL spectra of PFONPN01.
As shown in Fig. 2.7 (b), the absorption spectrum of DBT shows two peaks centred at around 306 nm and 446 nm, whereas the emission spectrum of PFONPN01 shows two peaks centred at around 415 nm and 441 nm. The excellent overlap between the PL spectrum of the PFONPN01 solid film and the absorption spectrum of DBT suggests efficient Förster energy transfer from the PFONPN01 host to the DBT guest. All these spectra were measured by spin casting DBT and PFONPN01 over a glass substrate. The film thickness used was ~ 80 nm.
Fig. 2.7 (a) The solid-state PL spectra of PFONPN01 and DBT and (b) the solid-state UV- visible absorption spectra of DBT and the solid-state PL spectra of PFONPN01 polymer.
In order to achieve white light, PFONPN01 was doped with di erent DBT concentrations in device W1 (0.2%), in device W2(0.4%) and in device W3(0.6%). Fig. 2.8 (a) and Fig. 2.8 (b) shows the current density vs. voltage (J–V) and brightness vs. current density (B–J) and curves of the fabricated devices. The key properties of all these WPLEDs are listed in Table 2.2. All the devices start emitting light at a voltage of approximately 5 V and are found to be highly bright. Device W3 exhibits a maximum brightnessvalue of 10180 cd m-2, whereas devices W1 and W2 possess a maximum brightness value of 8025 cd m-2 and 9565 cd m-2 respectively. The increase in the respective brightness of the WPLEDs as compared to that of the host material is due to the doping of narrow band gap DBT molecules. The addition of DBT introduces trap energy states inside the emissive
400 500 600 700
0.0 0.2 0.4 0.6 0.8 1.0
Normalized Intensity (a.u)
Wavelength (nm)
PFONPN01 DBT
200 300 400 500 600 700 0.0
0.2 0.4 0.6 0.8 1.0
Normalized Intensity (a.u)
Wavelength (nm)
UV DBT PL PFONPN01
(a) (b)
layers that act as a hole trap, resulting in a decrease in the hole mobility and current density.
Fig. 2.8 Current density vs. voltage (J-V) (a) brightness vs. current density (B-J) and (b) curves of the white PLEDs.
Fig. 2.9 The luminous efficiency vs. current density (LE–J) curves of WPLEDs.
Table 2.2 Key device properties of the fabricated white PLEDs.
Device Onset Voltage
(V)
Maximum Brightness B
(cd/m2)
Luminous Efficiencya
LE (cd/A)
Power Efficiencya PE (lm/W)
CIE Coordinate (x,y) at 10V
CRI
W1 8.16
(8.27, 0.09)
8025 (7814, 176.2)
6.07 (5.78, 0.36)
1.73 (1.66, 0.06)
0.26, 0.32 59
W2 8.56
(8.64, 0.06)
9565 (9205, 237.1)
6.54 (6.31, 0.21)
1.87 (1.80, 0.05)
0.31, 0.38 70
W3 8.90
(8.99, 0.07)
10180 (9910, 213.3)
6.37 (6.13, 0.20)
1.81 (1.73, 0.06)
0.30, 0.40 64
aMaximum value.Values in the bracket are average value of 10 devices and standard deviation.
0 2 4 6 8 10 12 14 16 0
50 100 150 200 250
Current Density (mA/cm2)
A Device W1 Device W2 Device W3
0 50 100 150 200 250
0.0 2.0k 4.0k 6.0k 8.0k 10.0k
Brightness(cd/m2)
Current Density (mA/cm2)
Device W1 Device W2 Device W3
(a) (b)
Voltage (V)
0 50 100 150 200 250
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
Luminous Efficiency (cd/A)
Current Density (mA/cm2)
Device W1 Device W2 Device W3
The luminous efficiency vs. current density (LE–J) curves of all the devices are shown in Fig. 2.9. Device W2 is found to be the most efficient with a maximum luminous efficiency of 6.54 cd A-1, whereas devices W1 and W3 have a maximum luminous efficiency of 6.07cd A-1 and 6.37 cd A-1 respectively. It was found that the device efficiency remains as high as ~ 3.25 cd A-1 with a luminance of over ~8025 cd m-2 for device W1, ~ 4.07 cd A-1 with a luminance of over ~ 9565 cd m-2 for device W2 and ~ 3.85 cd A-1 with a luminance of over ~ 10167 cd m-2 for device W3 at a current density of 250 mA cm-2. 2.3.4 Electroluminescence properties
Fig. 2.10 (a) shows the electroluminescence spectra of PFO, devices B4, W1, W2 and W3 taken at 10 V, whereas Fig. 2.10 (b) shows the chromaticity diagram representing the CIE coordinates of the fabricated PLEDs.
Fig. 2.10 The electroluminescence spectra of PFO, devices B4, W1, W2 and W3 taken at 10 V (a) and the chromaticity diagram representing the CIE coordinates of the fabricated PLEDs (b).
In EL spectra, we observed three peaks in the case of PFO centered at 436 nm, 463 nm and 493 nm. Another low intensity peak at around 530 nm is also observed which can be due to the excimers, interface species (exciplexes etc.) or keto defects.36–39 However, in the case of the PFONPN01 copolymer (device B4), the peak intensity at 464 nm gets reduced and the peak at 493 nm gets completely quenched. Instead, we get a peak at 486 nm. Hence, we assume that this peak is the result of the partial energy transfer from the PFO to the NPN unit. The EL spectra of devices W1, W2 and W3 show an additional peak
centered at around 550 nm with CIE coordinates of (0.26, 0.32), (0.31, 0.38) and (0.30, 0.40) respectively. The peak at around 550 nm can be attributed to the emission originating from the DBT molecule due to the Förster type energy transfer from the host matrix PFONPN01 to the dopants. The intensity of this peak is found to be altered with the DBT doping concentration. Device W2 with a 0.4% DBT doped PFONPN01 emissive layer is found to emit white light closest to the pure (0.33, 0.33) with a CIE coordinate of (0.31,0.38).